Rechargeable lithium and lithium-ion batteries are, due to their high energy density, widely used as power sources for portable electronic devices such as cellular phones, laptop computers, digital cameras or video cameras. Commercially available lithium-ion batteries typically consist of a graphite-based anode and an active cathode where lithium ions can reversibly be embedded and released.
Previously, LiCoO2 was the most used cathode material. However, LiCoO2-based cathode materials are expensive and typically have a relatively low capacity of about 150 mAh/g. Therefore, a substitution of LiCoO2 by materials such as lithium nickel oxide based cathodes (LNO), such as LiNi0.8Co0.2O2, nickel rich lithium nickel manganese cobalt oxides (LNMO), such as LiNi0.5Mn0.3Co0.2O2, or lithium nickel manganese cobalt oxides (LMNCO), such as LiNi0.33Mn0.33Co0.33O2, is in progress. However, an important concern with these layered oxides is their thermal instability in organic electrolytes. If the battery is charged, potentially, the delithiated cathode reacts with the electrolyte, creating heat which speeds up the reaction which ultimately might cause “thermal runaway” meaning the cell explodes. Doping of the cathode, if it lowers the reactivity of the cathode with electrolyte, can help to improve the safety of the cells. It is commonly known that aluminum can be doped into these cathode materials. It is also widely accepted that aluminum doping improves the safety properties of these materials. For example, whereas LiNi0.8Co0.2O2 is practically not applied due to a relatively high thermal instability, a related aluminum doped material (NCA) LiNi0.8Co0.15Al0.05O2, is commercially available. Generally, if aluminum is doped into a layered cathode material with a layer structure, the reversible capacity decreases by 1-2 mAh per mol % of aluminum. Thus, LiNi0.8Co0.2O2 has about 200 mAh/g reversible capacity at 4.3-3.0 V but 5% Al doped material (NCA) has about 190-194 mAh/g. This decrease in capacity, however, may be acceptable if the gain in improved safety is relatively significant.
Moreover, for applications where the energy density might be less important, for example for large size batteries like HEV or EV batteries, Li—Mn—O spinel and LiFePo4 based cathode materials are currently considered based on their better safety performance, despite their much lower energy density than above mentioned LNO, LNMO, and LMNCO materials.
Research has indicated that the solid solution “solubility” of aluminum in LNMCO cathode materials is relatively high, that the thermal instability decreases and therefore the safety increases relatively fast with an increasing aluminum doping level, and that relatively significant amounts of aluminum can be doped into LMNCO cathode materials while retaining a higher volumetric energy density than Li—Mn—O spinel or LiFePo4 based cathode materials. Considering these facts, it is obvious that aluminum doping with relatively high concentrations, for example >5 mol % Al/(Al+transition metal) may be a promising approach to achieve cathodes with superior performance compared to Li—Mn—O spinel and LiFePo4 based cathode materials.
A major problem, however, is that doping with aluminum is not a simple process. At production scale LNMCO cathodes are typically prepared from mixed metal precursors such as mixed transition metal hydroxide M(OH)2 or oxyhydroxide MOOH. The precursors are typically obtained by a precipitation of a base and acid solution, for example, 2NaOH+MSO4→M(OH)2+Na2SO4, possibly in the presence of a chelating agent like NH4OH. The precursor is then usually mixed with a lithium source (for example, Li2CO3) followed by a simple solid state reaction. While it is possible to dope aluminum into the precursor, the problem exists that aluminum does not fit easily into the M(OH)2 structure since the transition metal is divalent while aluminum is trivalent. As a result, instead of an M(OH)2 structure more complex structures such as layered double hydroxides, containing anionic impurities and crystal water, are obtained. It is further much more difficult to obtain a good morphology. For example, under conditions (such as temperature, pH, and so on) where M(OH)2 would precipitate with a good morphology, Al(OH)3 might be soluble causing a relatively poor morphology. Typical for co-precipitation with aluminum is a relatively low density and the obtained powder consists usually of unstructured fluffy agglomerates instead of nicely developed particles.
An alternative known approach is the coating with aluminum through a separate precipitation following the precipitation of the M(OH)2 structure. Under ideal conditions an Al(OH)3 layer will coat the M(OH)2 core. Such approach is described in EP1637503A1 where a lithium-nickel based cathode precursor is coated by an amorphous layer of aluminum hydroxide in a wet aluminum coating process. Wet aluminum coating is a relatively difficult process that often results in a relatively poor morphology since a sufficient density of the Al(OH)3 film might not be achieved. The existing impurity problem as described above may not be solved and it may be very difficult to achieve high doping levels of aluminum (>5 mol %) through wet aluminum coating due to the formation of relatively thick coating layers. Furthermore, wet aluminum coating is a relatively expensive process.
Coating of cathodes or cathode precursors has been described in the previous art. Dry coating by nano particles as fumed silica, fumed alumina, fumed zirconium, etc has been disclosed, but to our knowledge disclosures are limited to very small coating levels, typically not exceeding 1% by weight.
On a large scale precipitated metal hydroxides are in the known prior art mostly prepared by precipitating a base solution of NaOH with an acidic solution of MSO4 in the presence of a chelating agent, such as NH4OH. The obtained mixed metal hydroxide precursors, generally prepared by this cheap industrial route, contain impurities, which are undesired. The main impurities of interest are sulfate (typically 0.1-1 wt % SO42−), carbonate (typically 0.1-1 wt % CO32−), and sodium. The sulfate originates from the MSO4 and the carbonate originates (a) from carbonate impurities in the NaOH and (b) from CO2 in the air. As can be seen, it is relative difficult to avoid carbonate. The sulfate and carbonate impurities are positioned within the crystal structure and, therefore, may not be easily removed, for example by washing.
For preparing the final cathode material, such as LNMCO, lithium is added in the form of e.g. lithium carbonate or hydroxide. As lithium carbonate as well as lithium sulfate based salts are thermodynamically stable, the carbonate and sulfur impurities tend to remain in the final LNMCO cathode. A particular problem poses a carbonate impurity in Li—Ni-oxide based cathodes (such as LiNi0.8Co0.15Al0.05O2). The carbonate impurity causes poor high temperature performance of cells containing such cathode materials, such as swelling or bulging. A high quantity of sulfur impurities is possibly contributing to poor cathode performance and is, preferably, avoided. What is needed in the art is a coating process that enables the formation of particulate mixed transition metal oxide MO2, hydroxide M(OH)2 or oxyhydroxide MOOH precursors doped with aluminum that have an improved morphology and a lower impurity concentration compared to the known prior art.
It is a principal object of the present invention to provide novel precursors that enable the preparation of higher quality aluminum doped cathode materials, such as LNMCO or NCA cathode materials, at lower cost compared to currently available precursors.
In JP08-069902 a thermistor ceramic is disclosed, comprising a calcined manganese-nickel-copper-based oxide to which 0.1-20.0 wt % of aluminum oxide and/or zirconium oxide is added during grinding, after which the ground material is moulded and baked. In this method, neither aluminum oxide nor zirconium oxide forms a solid solution with the manganese-nickel-copper-based oxide, but is present in dispersed form at the grain boundary. There is no formation of an aluminum oxide coating on the ceramic.
In JP10-116603 a battery is disclosed that comprises: a positive electrode in which a manganese oxide is the active material, a negative electrode, and a non-aqueous electrolyte, and at least one type of additive selected from among Al2O3, In2O3, Ga2O3, Tl2O3, LiAlO2, LiInO2, LiGaO2 and LiTiO2 is added to the manganese oxide. However, in the formation of the positive electrode, according to the examples, alumina (Al2O3) was admixed with manganese dioxide (MnO2) which had been dehydrated at 400° C., in a proportion of 5 mol with respect to 100 atoms of manganese. The resulting mixture, carbon powder serving as a conducting agent, and a fluororesin powder serving as a binder were mixed in a weight ratio of 8:1:1 and compression moulded into a disc shape, and this was then subjected to heat-treatment at 250° C. in order to produce the positive electrode. By heat-treating such a mixture, alumina and carbon will compete to coat the manganese oxide, and it is impossible to form a continuous coating of alumina.